1. Field of the Invention
The present invention relates to an electrically controlled variable reflectance mirror and more particularly to an electrically controlled variable reflectance mirror formed with either a constant or graded reflectivity profile, suitable for use as an outcoupling mirror, for examples for use in a solid state lasing system, which enables the output of the lasing system to be controlled electronically and thus remotely to enable the solid state lasing system to be used in a relatively wide range of operating conditions with a single set of optics.
2. Description of the Prior Art
Solid state lasing systems are generally known in the art. Examples of such solid state lasing systems are disclosed in commonly-owned U.S. Pat. Nos. 5,555,254; 5,646,773; 6,094,297 and 6,178,040, hereby incorporated by reference. Such solid state lasing systems normally include an elongated slab of a solid state lasing material, for example, a rare earth doped yttrium-aluminum-garnet (YAG) crystal and one or more diode arrays for exciting the solid state lasing material to a relatively high metastable state. Exemplary diode arrays for pumping such solid state lasing systems are disclosed in U.S. Pat. Nos. 5,317,585; 5,351,251 and commonly-owned U.S. Pat. No. 6,208,677. Such solid state lasing systems also include a total reflectivity resonator mirror and an outcoupling mirror.
Various configurations of outcoupling mirrors and optical systems are known for outcoupling light from a lasing cavity. For example, variable reflective mirror systems are known to be formed from a static waveplate and a polarizer, as disclosed in “Solid State Laser Engineering”, by Walter Koechner, 2nd Edition, published by Springer-Verlag, page 201, copyright 1988. Such variable reflectance mirrors are also known to be formed from static birefringence elements and a polarizer as disclosed in “Solid State Engineering” by Walter Koechner, 2nd edition, published by Springer-Verlag, page 239, copyright 1988 and “Lasers”, by Siegman, published by University Science Books, page 921, copyright 1986. Such variable reflectivity mirrors are also discussed in detail in “Lasers” by Siegman, published by University Science Books, pages 913–922, copyright 1986 and U.S. Pat. Nos. 4,575,849; 5,260,964; 5,299,220 and 5,260,964.
U.S. Pat. No. 5,75,849 discloses a spatially varying optical filter which includes a spatially varying static birefringence element and a polarizer. U.S. Pat. No. 5,260,964 discloses a static graded reflectivity mirror formed from a partial reflector configured such that the reflectivity varies gradually as a function of the position on the mirror surface. U.S. Pat. No. 5,299,220 discloses a use of a one-dimensional static graded reflectivity mirror.
There is a trend toward multi-functioned solid state lasers that can operate over a relatively wide range of parameters, such as repetition rate, pulse energy and power. In known solid state lasing systems that are pumped by diode arrays, these parameters are typically varied by changing the way the lasing system is pumped. However, in order to optimize the performance of the lasing system, the outcoupling mirror system needs to be tailored to the gain corresponding to the specified parameters. Such static outcoupling mirror systems are unsuitable for easy systems that operate over a relatively wide range.
In order to meet this trend, both mechanical and electrically operated outcoupling mirror systems have been developed. For example, mechanical systems are known which include a static birefringence element and a polarizer in which the reflectance is varied by rotating the birefringence element. “Solid State Laser Engineering”, by Walter Koechner, 2nd Edition, published by Singer-Verlag, FIG. 5.28, page 201, copyright 1988. Such systems are costly and inefficient and require relatively precise alignment.
Electrically operated mirrors have been developed, used primarily in the automotive industry, to provide automatic reflectance control of rearview and side view mirrors as a function of ambient light intensity. Examples of such electrically controlled mirrors, also known as electrochromic mirrors, are disclosed in U.S. Pat. Nos. 5,066,112; 5,808,778 and 5,812,321. Unfortunately, such mirrors are unsuitable for use in lasing systems.
U.S. Pat. No. 6,041,071 (‘071’) discloses a variable reflectance mirror that is controlled electrically to enable the wavelength of a semiconductor laser system to be controlled externally. The electrically controlled variable reflectance mirror disclosed in the '071 patent is formed from a ferro-electric electro-optical substrate and a distributed Bragg reflector. An external voltage applied to the ferro-electric electro-optical substrate controls the strain and thus the characteristics of the distributed Bragg reflector to enable the wavelength of the output beams passing through the outcoupling mirror to be electrically controlled. Unfortunately, such a system is not suitable for electrically controlling other characteristics, such as the output power of a lasing system. Accordingly, there is a need for an outcoupling mirror system whose reflectivity can be electrically controlled to enable a lasing system to be used with different operating conditions with a single set of optics.